Factorial scopes of cardio-metabolic variables remain constant with changes in body temperature in the varanid lizard, Varanus rosenbergi
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The majority of information concerning the cardio-metabolic performance of varanids during exercise is limited to a few species at their preferred body temperature (T-b) even though, being ectotherms, varanids naturally experience rather large changes in T-b. Although it is well established that absolute aerobic scope declines with decreasing T-b, it is not known whether changes in cardiac output (V(over dot)(b)) and/or tissue oxygen extraction, (Ca-O2 - Cv(over dot)(O2)), are in proportion to the rate of oxygen consumption (V(over dot)(O2)). To test this, we studied six Rosenberg's goannas (Varanus rosenbergi) while at rest and while maximally exercising on a treadmill both at 25 and 36 degrees C. During maximum exercise both at 25 and 36 degrees C, mass-specific rate of oxygen consumption (V(over dot)O-2kg) increased with an absolute scope of 8.5 ml min(-1) kg(-1) and 15.7 ml min(-1) kg(-1), respectively. Interestingly, the factorial aerobic scope was temperature-independent and remained at 7.0 which, at each T-b, was primarily the result of an increase in V(over dot)(bkg), governed by approximate twofold increases both in heart rate (f(H)) and cardiac stroke volume (V-Skg). Both at 25 degrees C and 36 degrees C, the increase in V(over dot)(bkg) alone was not sufficient to provide all of the additional oxygen required to attain maximal V(over dot)(O2kg), as indicated by a decrease in the blood convection requirement V(over dot)(bkg)/V(over dot)O-2kg; hence, there was a compensatory twofold increase in (Ca-O2 - Cv(over dot)(O2)). Although associated with an increase in hemoglobin- oxygen affinity, a decrease in T-b did not impair unloading of oxygen at the tissues and act to reduce (Ca-O2 - Cv(over dot)(O2)); both Ca-O2 and Cv(over dot)(O2) were maintained across T-b. The change in V(over dot)(O2kg) with T-b, therefore, is solely reliant on the thermal dependence of V(over dot)(bkg). Maintaining a high factorial aerobic scope across a range of T-b confers an advantage in that cooler animals can achieve higher absolute aerobic scopes and presumably improved aerobic performance than would otherwise be achievable.
|Journal||AJP Regulatory Integrative and Comparative Physiology|
|Publication status||Published - 1 Apr 2005|
- oxygen extraction, heart rate, metabolic rate, exercise, stroke volume, goanna, reptile, Fick equation, oxygen consumption, cardiac output